HK scientists find new way to spot fetal abnormalities

PUBLISHED : Friday, 06 May, 2011, 12:00am
UPDATED : Friday, 06 May, 2011, 12:00am


Hong Kong researchers have produced a pioneering technology that allows pregnant women to find out in just one week if their unborn child has chromosome abnormalities that would result in severe disabilities.

The Fetal DNA Chip, developed at the Chinese University, adapts a technique used in screening tumours to track chromosome changes in the unborn child.

Known as comparative genomic hybridisation it can spot abnormalities that are invisible to the conventional approach, karyotype analysis, which uses microscopes.

First used in 2009 at the prenatal genetic diagnosis centre of the university's department of obstetrics and gynaecology, it has since been used in 281 cases before being introduced to the public yesterday.

'We developed this chip because we wanted to introduce a more comprehensive search for genetic abnormalities with small DNA regions,' Professor Richard Choy Kwong-wai, an associate professor in the obstetrics department, said.

The centre is the only place using the technology in a clinical setting.

It was developed after researchers found that an indicator for Down's syndrome - an increase in fluid under the skin in the nuchal area at the back of the baby's neck - could indicate the presence of other abnormalities too tiny to be detected by a microscope.

The Hospital Authority has offered free Down's screening since July last year.

This uses ultrasound to measure the fluid, known as a nuchal translucency (NT) test. If an increase is found, fetal karyotyping is recommended to confirm the condition.

Using the new technique, the university's researchers tested 48 samples and found that 8.3 per cent of fetuses with a nuchal translucency of 3.5mm or above carried chromosomal aberrations that were either fatal or associated with severe congenital abnormalities, that would have otherwise been missed.

They found that more than 100 diseases were associated with an increased NT.

'Think of DNA as string tightly wound together,' said Professor Lau Tze-kin, an honorary professor with the department. 'What we are looking for is when chunks of this are missing. We are looking for smaller deletions or additions of chromosomes that are not as noticeable as Down syndrome, which can result in conditions like Digeorge syndrome.'

Digeorge syndrome can cause recurrent infection, heart defects and distorted facial features.

The test involves taking a sample of fetal DNA from the womb.

DNA segments known as props are then hybridised from the corresponding DNA sections in the sample, along with the a sample from a person with no chromosome abnormalities.

'We scan these into a computer,' Lau said. 'On the genomes where we have put a prop there is a high resolution ... the software recognises this and shows us which of the DNA segments are balanced, which have extra chromosomes and which have fewer than there should be.' In the case of an abnormality, 'we explain the relevant disorder to the parents [and] advise them on the financial and development implications,' Lau said. 'We will never say whether to carry on or terminate the baby.'

The new method is also faster, yielding results in seven days while the microscope method can take three times as long.

But it is more expensive at HK$7,900, compared to HK$2,000 for karyotype analysis.

Private obstetrician Dr Kun Ka-yan, who was not involved in the research, said the test should be applied but that money was a concern.

'The Hospital Authority will not pay for it, so to do it, the client will have to pay,' Kun said.

How the testing process works

1 Baby, mother and father provide blood samples. This patient DNA and another standard DNA sample are labelled with fluorescent dyes.

2 The DNA samples are applied to a microarray, a chip used to show up differences between the standard DNA and a patient?s.

3 The DNA samples hybridise, or combine, on the microarray.

4 Leaving differences between the samples showing up by the coloured dyes.

5 These colour signals can be analysed by computer software, indicating where DNA has been lost or gained by the patient, which indicates possible illness or a debilitating condition in the unborn baby.